As a conventional type of image forming apparatus based on an electrophotographic system, there is, for instance, an image forming apparatus in which a plurality of toner images of gradated pattern are formed on an image carrier, a quantity of toner deposited on each toner image is detected by an electrophotosensitive chip, and the image signal is corrected according to the result of detection to prevent degradation of the image such as dirt of image due to such causes as change with the lapse of time of a developer or changing in development characteristics because of effects by the surroundings or a low image density. Concretely, the conventional type of image forming apparatus as described above is disclosed, for instance, in Japanese Patent Laid Open Publication No. 260067/1985 disclosing "Electrophotography controlling method" or Japanese Patent Laid Open Publication No. 126462/1992 disclosing an "Image forming apparatus".
However, in an image forming apparatus in which a quantity of deposited toner is detected by using a conventional type of electrophotosensitive chip, a quantity of toner deposited on a image carrier becomes larger, and when the quantity exceeds a value specified previously, and output value from the electrophotosensitive chip becomes saturated and the sensitivity in detection becomes lower, so that measurement of a quantity of deposited toner on the image carrier becomes impossible when the quantity exceeds the prespecified value.
Also in the conventional type of image forming apparatus, as an image signal is corrected according to a quantity of deposited toner detected as described above, the image signal can be corrected, when a quantity of deposited toner is relatively lower, so that a desirable image density can be obtained, but when a quantity of deposited toner is relatively higher and accordingly a sensitivity of an electrophotosensitive chip is low, sometimes it is impossible to correct an image signal for obtaining a desired image density.